Conventionally, a head-up display device (hereinafter abbreviated as HUD device) configured to virtually display an image to be visible to an occupant has been known. The HUD device disclosed in Patent Literature 1 includes an illumination light source unit, an image display panel, and a projection lens. The illumination light source unit emits an illumination light. The image display panel causes the illumination light from the illumination light source unit side to pass through the image display panel and to be emitted from a display surface as a display light to display an image. The projection lens is located between the illumination light source unit and the image display panel, and projects the illumination light from the illumination light source unit side onto the image display panel.
In Patent Literature 1, the image display panel is located so that an optical axis of the illumination light source unit coincides with a normal direction of the display surface. Further, the projection lens is located so that the optical axis is orthogonal to a radial direction of the projection lens.
The present inventor has considered that the image display panel is tilted so that the normal direction of the display surface intersects with the optical axis. According to the tilted image display panel, even when an external light such as sunlight enters the image display panel in a direction opposite to the display light, since the normal direction of the display surface intersects with the external light, the external light is restricted from being reflected by the display surface and visually recognized together with the display light.
On the other hand, the present inventor has found out that the following unique issue arises in the HUD device in which the projection lens for projecting the illumination light onto the tilted image display panel is located such that a radial direction of the projection lens is orthogonal to the optical axis. More specifically, when the projection lens and the image display panel are placed on an optical path with the avoidance of mutual interference between the projection lens and the image display panel, an interval between the projection lens and the image display panel is partially widened due to an angular difference of the placement, and a dead space may occur between the projection lens and the image display panel. As a result, for example, a size of the HUD device increases due to an increase in a distance from the illumination light source unit to a tip of the image display panel. In other words, the mountability of the HUD device to a movable object is deteriorated.